1. Field of the Invention
The present invention relates to an optical scanning apparatus for one-dimensionally scanning an object by light and more particularly to an optical scanning apparatus for use in a laser bar code reader, a laser printer, a laser facsimile, a laser text reader, a digital copier and the like.
2. Description of Related Art
A laser scanner is an apparatus for scanning an object by laser light. In general, the laser scanner is constructed so as to reflect laser light emitted from a laser oscillator by a scanning mirror which rotates or reciprocates and to one-dimensionally scan the surface of an object by the reflected light. A correcting lens or mirror is provided between a predetermined plane of the object and the scanning mirror in order to irradiate laser light in focus and to scan the object at a predetermined scanning speed. A direction in which the laser light is irradiated is changed and the scanning speed is corrected by causing the reflected light to pass through the correcting lens or by reflecting the reflected laser light again by the mirror. Or, thereby, a focal position is changed.
FIG. 10 is a diagram showing a structure of a typical laser scanner according to a first prior art technology. The laser scanner comprises a laser oscillator 31, a condenser lens 32, a one plane mirror 33, imaging lenses 35, a light receiving element 36 and a return mirror 37. The one-plane mirror 33 is a scanning mirror for deflecting a laser beam so that it scans within a predetermined scanning range. It has a single reflecting plane and is provided with an axis of rotation within the reflecting plane so as to be angularly displaceable. The laser beam emitted from the laser oscillator 31 is condensed by the condenser lens 32 to be reflected in an arbitrary direction by the one plane mirror 33. The reflected laser beam is imaged on a scanning position 34 on a predetermined line segment within the plane by the imaging lens 35 which is a f.theta. lens. The return mirror 37 reflects the laser beam imaged out of the above-mentioned line segment among the laser beams which have passed through the imaging lens 35 so as to guide to the light receiving element 36. The light receiving element 36 is disposed right before the scanning position 34 to detect the position of the laser beam which has passed through the imaging lens 35.
FIG. 11 is a diagram showing a structure of a typical laser scanner according to a second prior art technology. The structure of this laser scanner is the same with the laser scanner shown in FIG. 10, except of that the imaging lens 35 is replaced by a curved reflecting mirror 38. The curved reflecting mirror 38 allows the same effect with that of the imaging lens 35 to be obtained and its reflecting plane is formed by a spherical plane whose radius of curvature in the main scanning direction of the laser beam is fixed. In this laser scanner, the laser beam reflected by the one plane mirror 33 is reflected again by the curved reflecting mirror 38 to image at the scanning position 34. A number of proposals have been made since the past as to the concrete structure of the lenses and mirrors composing the laser scanners shown in FIGS. 10 and 11.
While an object scanning speed which is a speed at which the laser beam scans the surface of the object may be increased by increasing the speed of rotation of the one plane mirror 33 in the laser scanners shown in FIGS. 10 and 11, there is a limit in the increase of the speed of rotation because the faster the speed of the rotation, the more complicated the structure of a mechanism for rotating the one plane mirror 33 becomes and the harder the control of the rotation thereof becomes. Therefore, a polygon mirror having a plurality of reflecting planes is normally used instead of the one-plane mirror in increasing the object scanning speed. When the polygon mirror is used, however, the position of reflecting point in the reflecting plane where the laser beam is reflected changes corresponding to an angle of rotation of the polygon mirror because the center of rotation of the polygon mirror is separated considerably from the reflecting planes. The variation of the position of the reflecting point varies depending on the size of an inscribed circle of the polygon mirror and the larger the inscribed circle, the greater the variation becomes.
FIG. 12 is a diagrammatic view for explaining an angular displacement of the reflecting plane of the polygon mirror and changes of the reflecting point. The polygon mirror 39 is assumed to be rotated in the direction indicated by an arrow A centering on the center of rotation C.
In a first state when the laser beam starts to enter one reflecting plane of the polygon mirror 39, the reflecting plane is positioned so that the laser beam 20 condensed by the condenser lens 32 enters the reflecting plane with an incident angle of 45 degrees or more as indicated by a solid line 39a. In the first state, the laser beam 20 reflects at a reflecting point P1 and is deflected in the direction indicated by an arrow 21.
When the polygon mirror 39 is rotated further from the first state, the reflecting plane is positioned so that the laser beam 20 enters the reflecting plane with an angle of 45 degrees as indicated by a two-dot chain line 39b. In the second state, the laser beam 20 reflects at a reflecting point P2 and is deflected in the direction indicated by an arrow 22. The reflecting point P2 is located on an imaginary axis containing an optical path of the laser beam 20 and behind the reflecting point P1 in the incident direction of the laser beam 20. Therefore, the reflecting point moves from the reflecting point P1 to the reflecting point P2 in the direction indicated by an arrow a1 during when the polygon mirror 39 is rotated so as to put the reflecting plane into the second state from the first state.
When the polygon mirror 39 rotates further from the second state, the reflecting plane is positioned so that the laser beam 20 enters the reflecting plane with an incident angle of less than 45 degrees as indicated by a broken line 39c. The laser beam 20 reflects again at the reflecting point P1 and is deflected in the direction indicated by an arrow 23 in this third state. Therefore, when the polygon mirror 39 rotates so as to put the reflecting plane into the third state from the second state, the reflecting point moves from the reflecting point P2 to the reflecting point P1 in the direction indicated by an arrow a2. When the polygon mirror 39 is rotated so as to put the reflecting plane into the first through third states sequentially, the reflecting point reciprocates on the imaginary axis.
The reflecting point will not move as described above when the axis of rotation of the one plane mirror 33 is made to coincide with the reflecting plane in rotating the one plane mirror 33 because the reflecting point within the reflecting plane is always fixed. Therefore, a range in which the laser beam reflected by the one plane mirror can be deflected is bisected by the laser beam which is reflected by the one plane mirror 33 at 45 degrees. Because the reflecting point moves when the one plane mirror 33 of the laser scanner shown in FIGS. 10 and 11 is replaced by the polygon mirror 39 as described above, the linearity of scan at the scanning position 34 corrected by the imaging lens 35 or the curved reflecting mirror 38 drops considerably as compared to the case of using the one plane mirror 33. Further, thereby, the range in which the laser beam reflected by the polygon mirror 39 can be deflected will not be bisected even if the laser beam which is reflected by the polygon mirror 35 at 45 degrees is set as a boundary.
For instance, in the above-mentioned range, a first range to which the laser beam whose reflecting angle at the polygon mirror 39 is 45 degrees or more is emitted, i.e. a range from the direction indicated by the arrow 21 to the direction indicated by the arrow 22, is narrower than a range to which the laser beam whose reflecting angle at the one plane mirror 33 is 45 degrees or more is emitted. Conversely, a second range to which the laser beam whose reflecting angle at the polygon mirror 39 is 45 degree or less is emitted, i.e. a range from the direction indicated by the arrow 22 to the direction indicated by the arrow 23, is wider than the range to which the laser beam whose reflecting angle at the one plane mirror 33 is 45 degrees or less is emitted.
Accordingly, if the speed of rotation of the reflecting plane of the polygon mirror 39 is equal with the speed of rotation of the reflecting plane of the one plane mirror 33, the speed for scanning the curved reflecting mirror 38 by the laser beam emitted in the first range is slower than the speed for scanning the curved reflecting mirror 38 by the laser beam reflected by the one plane mirror 33, bounding the laser beam whose reflecting angle at the polygon mirror 39 is 45 degrees. Conversely, the speed for scanning the curved reflecting mirror 38 by the laser beam emitted to the second range is faster than the speed for scanning the curved reflecting mirror 38 by the laser beam reflected by the one plane mirror 33. The object scanning speed in scanning the object by reflecting those laser beams by the curved reflecting mirror is proportional to the scanning speed of the curved reflecting mirror 38. From these facts, when the polygon mirror is used as the scanning mirror, the object scanning speed is not kept at a predetermined speed, producing a large error in the scanning speed. Therefore, a f.theta. correction error may increase.
As a prior art technology of this laser scanner, there has been an optical scanning apparatus using an optical scanning curved mirror as disclosed in Japanese Unexamined Patent Publication JP-A 62-253116 (1987). This optical scanning apparatus is constructed so as to deflect light emitted from a light source by a scanning mirror and to reflect this light by an optical scanning curved mirror to condense and image linearly on a photoreceptor drum. A curved profile of this optical scanning curved mirror in the direction orthogonal to a locus on which a reflecting point where the light deflected by the scanning mirror is reflected moves is slanted such that an incident angle of the light increases gradually from the middle part to the both ends in order to deflect the light deflected by the scanning mirror in the direction intersecting with the plane of deflection of the light. Because the curved profile in the direction parallel with the locus in the reflecting planes of the optical scanning curved mirror is nearly parabolic, the radius of curvature thereof is almost fixed. Therefore, it is difficult to prevent the error of the object scanning speed described above.